2018ApJ...859....8L -
Astrophys. J., 859, 8-8 (2018/May-3)
A candidate tidal disruption event in a quasar at z = 2.359 from abundance ratio variability.
LIU X., DITTMANN A., SHEN Y. and JIANG L.
Abstract (from CDS):
A small fraction of quasars show an unusually high nitrogen-to-carbon ratio (N/C) in their spectra. These "nitrogen-rich" (N-rich) quasars are a long-standing puzzle because their interstellar medium implies stellar populations with abnormally high metallicities. It has recently been proposed that N-rich quasars may result from tidal disruption events (TDEs) of stars by supermassive black holes. The rapid enhancement of nitrogen and the depletion of carbon due to the carbon-nitrogen-oxygen cycle in supersolar mass stars could naturally produce high N/C. However, the TDE hypothesis predicts that the N/C should change with time, which has never hitherto been observed. Here we report the discovery of the first N-rich quasar with rapid N/C variability that could be caused by a TDE. Two spectra separated by 1.7 years (rest-frame) show that the N III] λ1750/C III] λ1909 intensity ratio decayed by ∼86% ± 14% (1σ). Optical (rest-frame UV) light-curve and X-ray observations are qualitatively consistent with the TDE hypothesis; though, the time baseline falls short of a definitive proof. Putting the single-object discovery into context, statistical analyses of the ∼80 known N-rich quasars with high-quality archival spectra show evidence (at a 5σ significance level) of a decrease in N/C on timescales of >1 year (rest-frame) and a constant level of ionization (indicated by the C III] λ1909/C IV λ1549 intensity ratio). If confirmed, our results demonstrate the method of identifying TDE candidates in quasars via abundance ratio variability, opening a new window of TDE observations at high redshift (z > 2) with upcoming large-scale time-domain spectroscopic surveys.
Abstract Copyright:
© 2018. The American Astronomical Society. All rights reserved.
Journal keyword(s):
accretion, accretion disks - black hole physics - galaxies: abundances - galaxies: active - galaxies: nuclei
VizieR on-line data:
<Available at CDS (J/ApJ/859/8): table2.dat>
Simbad objects:
5
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